Environmental and genetic factors which influence transposition of Tn9 in E. coli have been studied. Acetate ion and several lipid solvents (Brij 58, NP40 and dimethylsulfoxide) increase the frequencies of Tn9, Tn5 and Tn10 transpositions. The C16 fatty acid, palmitic acid, reverses the effect of Brij 58. Compared to wild-type, a rho mutant (defective in transcription termination) shows a 20 fold lower frequency of transposition for Tn9 but is normal for Tn5 and Tn10. Strains deleted for the gal-attlambda region of the chromosome are defective in a special transposition event wherein an IS1 or IS5, transpose to a distant chromosomal site (bg1).